The present invention relates generally to precompressing ferroelectric plates, and more particularly to a method of electrically inducing mechanical precompression in ferroelectric plates used as sound projectors.
The use of ferroelectric ceramic materials as electrical-mechanical transducers is well known in the art. In particular, such materials are used as underwater sound transducers, where mechanical resonances of a ferroelectric ceramic plate generated by an applied ac voltage at certain frequencies are used to transmit sound. One such use is shown in applicant's U.S. Pat. No. 4,524,295, where mechanical resonances of a plate are used to generate mechanical waves.
Plates driven into flexural resonances by applied electrical signals are vibrated through the plane in which they lie at rest. For such simply supported plates vibrating in the fundamental mode, one side is alternately placed under tension and then compression, while the opposing side is under compression and then tension. The mechancial properties of the plate determine the maximum amplitude of vibration the plate may undergo without breaking. Since ceramics can withstand large compressive forces, but not large tensile forces, the failure is typically a tensile failure.
It is well known in the art to prestress materials such as ferroelectric ceramics to increase their power-handling capabilities. Prior prestressing techniques have relied on mechanically altering the material or using external agents. For example, U.S. Pat. No. 3,588,552 of Hugo Schafft applies a dc voltage to polarize a weighted disk to snap it into a retaining ring having a slightly smaller inside diameter than the outside diameter of the disk. U.S. Pat. No. 3,706,967 of Nicholas Renna prestresses piezoelectric staves by wrapping them with fiberglass under tension.
A disadvantage of known methods of prestressing ferroelectric ceramics is their use of mechanical processes; thereby increasing the degree of skill and cost of their manufacture. Applicant has reported on the mechanical responses of a ferroelectric ceramic to a slowly varying cyclic electric field (Int. J. Engng Sci. Vol. 19, 1981, pp. 147-158). This invention utilizes these responses to electrically prestress a ferroelectric ceramic.